1. Field of the Invention
The present invention relates to methods for processing milk, and more particularly pertains to a new process for removing, or reversing, an oxidized off-flavor from milk.
2. Description of the Prior Art
Milk may acquire an off-taste in a number of ways. For example, under proper conditions, unsaturated fatty acids that are present in milk fat can react with oxygen to form oxidation products, and the presence of these oxidation products in the milk can give an “oxidized off-flavor” to the milk. More specifically, milk that has an oxidized off-flavor will exhibit a cardboard-like, metallic, or tallowy taste and smell (Bodyfelt, F. W., J. Tobias, and G. M. Trout, 1988, The Sensory Evaluation of Dairy Products, Van Nostrand Reinhold, New York, N.Y.). The rate of oxidation of these unsaturated fatty acids can be affected by many factors, including the amount of oxygen present in the environment, the degree of unsaturation of the lipid in the milk, the presence of antioxidants or prooxidants in the milk, the nature of the packaging material for the milk, the temperature of the storage of the milk, and the degree of light exposure to the milk (deMan, J. M., 1980, Principles of Food Chemistry, The AVI Publ. Co., Inc., Westport, Conn.). “Metal-induced” oxidized off-flavor in milk was once a common problem caused by metal contamination of the milk, which resulted from exposure to dairy equipment that contained copper. Today most dairy equipment is manufactured from stainless steel, which does not cause metal contamination of milk and thus stainless steel does not contribute to a metal-induced oxidized off-flavor in milk. (Henderson, J. L., 1971, The Fluid-Milk Industry, 3rd ed., The AVI Publ. Co., Inc., Westport, Conn.). Despite this change in equipment, the problem of the oxidized off-flavor in milk has not disappeared.
In 1962, only about 3% of the fluid milk produced in the U.S. was packaged in plastic containers, while today about 81% of the fluid milk is packaged in plastic containers. Plastic containers that are clear in character easily transmit ultraviolet light (e.g., light waves in the 350 to 500 nanometer range) which is damaging to milk nutrients and can cause “light-induced” oxidized off-flavor (also known as light-activated flavor) in the milk (White, C., 1984, Dairy Record 85(4): 122). Ultraviolet light is produced by fluorescent lights, which are found in most dairy display cases in supermarkets in the United States. The time of light exposure that produces light-induced oxidized off-flavor in milk is dependent upon the distance of the milk container from the fluorescent light. It is estimated that for large supermarkets, the turnover rate of milk in the display case ranges from approximately 4 to 8 hours and the typical range of fluorescent light intensity is between 750 and 1000 lux (White, C., 1984, Dairy Record 85(4):122). Exposure of milk to fluorescent light for extended periods of time can also cause the loss of vitamins A, riboflavin (B2), and C from the milk (Hansen, A., 1998, Hoard's Dairymen 143(4):147).
Several approaches have been reported for attempting to prevent or inhibit the acquisition of an oxidized off-flavor by milk. The reduction in the per capita consumption of milk in the U.S. has been attributed to the increased use of plastic milk containers and increased incidence of light-induced oxidized off-flavor (Hansen, A., 1998, Hoard's Dairymen 143(4):147). Paper milk cartons typically provide greater protection for milk from light-induced oxidized off-flavor than clear glass or clear plastic. Opaque or pigment-modified containers can inhibit light-induced oxidized off-flavor. Such containers have been developed by adding light-blocking agents to the resins before blow molding single-service plastic bottles (Bodyfelt, F. W., J. Tobias, and G. M. Trout, 1988, The Sensory Evaluation of Dairy Products, Van Nostrand Reinhold, New York, N.Y.). Today some dairy plants are offering milk in opaque colored bottles that protect the flavor and nutrients of the milk (Broihier, K., 1998, Food Processing 60(3):53). These containers do appear to offer better protection from light than clear plastic containers, however some light can still be transmitted through the wall of the container.
Other factors may minimize or reduce the formation of the oxidized off-flavor in milk. Homogenization of milk increases the surface area of the fat globules in the milk, and the composition and the properties of the fat globules change. The new membrane that is formed around the fat globule consists primarily of protein which protects the phospholipids in the milk fat from attack by oxygen. This mechanism can give the homogenized milk some protection against the development of a “metal-induced” oxidized off-flavor. However, the homogenized milk is more susceptible to “light-induced” oxidized off-flavor (Bodyfelt, F. W., J. Tobias, and G. M. Trout, 1988, The Sensory Evaluation of Dairy Products, Van Nostrand Reinhold, New York, N.Y.). High heat treatment (at 76° C. to 78° C.) of milk provides protection against the development of an oxidized off-flavor. Sulfide and sulfhydryl groups from whey proteins are liberated or activated, which can act as antioxidants (Gould, I. A. and H. H. Sommer, 1939, Michigan Agr. Exp. Stn. Tech. Bull. 164). Another factor which may control the formation of an oxidized off-flavor in milk is the introduction of green feed into the dairy cow rations (Bodyfelt, F. W., J. Tobias, and G. M. Trout, 1988, The Sensory Evaluation of Dairy Products, Van Nostrand Reinhold, New York, N.Y.). Also, increasing the tocopherol (vitamin E) content in the milk can inhibit the formation of an oxidized off-flavor in the milk (Krukovsky, 1949, J. Dairy Sci. 32:196.
The oxidative reaction, once started is a chain reaction. New free radicals are produced that continue the reaction and oxidized off-flavor becomes stronger with time. Aldehydes, ketones, and methional are compounds that typically cause oxidized off-flavors. “Metal-induced” oxidized off-flavor has been described as a cardboard-like or tallowy taste, and “light-induced” oxidized off-flavor has been described as a cabbage-like, chemical-like, burnt protein, burnt feathers, burnt plastic, or mushroom-like taste (Bodyfelt, F. W., J. Tobias, and G. M. Trout, 1988, The Sensory Evaluation of Dairy Products, Van Nostrand Reinhold, New York, N.Y.). It has been postulated that when the amino acid methionine is exposed to light in the presence of riboflavin (vitamin B2), which is also present in milk, a degradation occurs which leads to the formation of methional. Methional is an important compound that contributes to “light-induced” oxidized off-flavor (Patton, S., 1954, J. Dairy Sci. 37:446).
While a number of studies have reported the inhibition or partial inhibition of the formation of a “light-induced” oxidized off-flavor in milk, none of the known studies have reported the reversal of a light-induced oxidized off-flavor in milk once the off-flavor has been formed in the milk.
Thus, while the known literature has dealt to some extent with attempts to prevent or mitigate the formation of the an oxidized off-flavor in milk, the techniques described in the literature fail to address the reduction or elimination of the oxidized off-flavor in milk once it has occurred. Therefore, there is a need for a method for removing or reversing the oxidized off-flavor in the milk once it has occurred.